Heat pump assisted distillation. V: A feasibility study on absorption heat pump assisted distillation systems

The problems in matching a heat driven absorption heat pump to a distillation process in heat pump assisted distillation are discussed. The performance of an absorption system is a function of the temperatures in the evaporator, the condenser, the absorber and the generator and the ratio of the mass flow rate in the secondary circuit to the mass flow rate in the primary circuit. In absorption systems design choices are limited by the Gibbs phase rule. Plots are given of the coefficient of performance against the temperatures of the top and bottom products and also against the energy saved.     

Heat pump assisted distillation. IV: An experimental comparison of R114 and R11 as the working fluid in an external heat pump

An experimental study has been carried out on a continuously operated pilot fractional distillation column equipped with an external heat pump. The distillation column was a 15 cm internal diameter glass unit containing eleven single bubble cap plates. A methanol-water mixture was fed to the column. After operating the heat pumps with R114 as the working fluid, further experiments have been conducted with R11 as the working fluid. Plots of pressure against enthalpy, condensation pressure and latent heat of vaporization against condensation temperature and theoretical Rankine coefficient of performance against gross temperature lift and condensation temperature are presented for both R114 and R11. R11 has correspondingly higher theoretical Rankine coefficients of performance than R114. The experiments show that the actual coefficients are also higher for R11 than for R114. A maximum actual coefficient of performance of 5.3 was obtained using R11 as the working fluid with a gross temperature lift of 38.4°C. The experimental data for R11 were found to be reproducible during operation over a number of weeks. This showed that the relative thermal instability of R11 compared to R114 had not apparently affected the performance of the system.     

Heat pump assisted distillation. III: Experimental studies using an external heat pump

An experimental study has been carried out on a continuously operated pilot fractional distillation column equipped with an external heat pump. The distillation column was a 15 cm diameter glass unit containing eleven single bubble cap plates. A methanol-water mixture was fed to the column and the heat pump working fluid was R114. The actual coefficient of performance (COP)_A of the heat pump increased with an increase in the mass flow rate of the working fluid. A maximum (COP)_A value of 4–3 was obtained with a gross temperature lift of 41–3°C. The performance of two reciprocating compressors was compared. The experiments have shown that continuous heat pump assisted distillation using an external working fluid can greatly reduce the energy used in a distillation process. No control problems were encountered in the experiments.     

Increase of COP for heat transformer in water purification systems. Part I – Increasing heat source temperature

The integration of a water purification system in a heat transformer allows a fraction of heat obtained by the heat transformer to be recycled, increasing the heat source temperature. Consequently, the evaporator and generator temperatures are also increased. For any operating conditions, keeping the condenser and absorber temperatures and also the heat load to the evaporator and generator, a higher value of COP is obtained when only the evaporator and generator temperatures are increased. Simulation with proven software compares the performance of the modeling of an absorption heat transformer for water purification (AHTWP) operating with water/lithium bromide, as the working fluid–absorbent pair. Plots of enthalpy-based coefficients of performance (COP_ET) and the increase in the coefficient of performance (COP) are shown against absorber temperature for several thermodynamic operating conditions. The results showed that proposed (AHTWP) system is capable of increasing the original value of COP_ET more than 120%, by recycling part of the energy from a water purification system. The proposed system allows to increase COP values from any experimental data for water purification or any other distillation system integrated to a heat transformer, regardless of the actual COP value and any working fluid–absorbent pair.     

Experimental study of the operating characteristics of a heat pump assisted distillation system using R11 as the external working fluid

Heat pump assisted distillation with an external working fluid is one of the most obvious methods to reduce the energy consumption in a distillation process. The heat pump working fluid extracts heat from the top of the column, increases the temperature of the recovered heat and recycles it to provide the heat input to the reboiler.The interaction between the external parameters and the internal parameters for a specially designed heat pump assisted distillation system has been studied experimentally. The external parameters were mass flow rate, temperature and concentration of the feed, the concentration of the top and bottom products and the mass flow rate of the working fluid. The effects of the variations of these external parameters on the internal parameters such as the energy (steam) consumption, the actual coefficient of performance and the temperatures at the top and bottom of the column, together with the condensation and evaporation temperatures, are presented.     

Heat pump assisted distillation. IX: Acceptance trials on a system for separating ethanol and water

An experimental study has been carried out on a continuously operated pilot fractional distillation column equipped with an external heat pump. The distillation column was a 153 mm diameter stainless steel unit containing fourty-four sieve plates. An ethanol-water mixture was fed to the column and the heat pump working fluid was R114. The actual coefficient of performance (COP)_A of the heat pump increased with an increase in the mass flow rate of the working fluid. A maximum value of 4.5 was obtained with a gross temperature lift of 45°C. The performance of two reciprocating compressors was compared. A heat pump effectiveness factor of 0.8 was achieved. A maximum relative contribution of the heat pump of 65 per cent was obtained with minimum temporary insulation. With good insulation it is estimated that the relative contribution of the heat pump should exceed 80 per cent at the design feed rate. No control problems were encountered in the experiments.     

Heat pump assisted distillation. II: Matching an external heat pump to a fractional distillation system

The problems in matching an external mechanical vapour compression heat pump to a distillation process in heat pump assisted distillation are discussed. There are four main design parameters to consider when selecting a working fluid for the heat pump, of which only two are independent. Various arrangements for imperfectly matched systems are presented, including the use of a two stage heat pump. A number of other factors which affect the choice of a heat pump assisted distillation system, including economic factors, are also considered.     

Heat pump assisted distillation. I: Alternative ways to minimize energy consumption in fractional distillation

A survey is made of the numerous possible ways to minimize the consumption of energy in fractional distillation. Distillation is a highly energy-intensive process in which the actual energy consumed is far greater than the theoretically calculated loss in available work. The methods considered for reducing energy consumption are classified as those which (i) do not require process modifications, (ii) require minor process modifications, (iii) make use of more than one column and (iv) make use of heat pumps.     

Heat pump assisted distillation. VI: Classified references for heat pump assisted distillation systems from 1945 to February 1986

More than one hundred and fifty references for heat pump assisted distillation systems have been listed and classified under the following categories: heat pump assisted distillation (1) with an external working fluid, (2) using one of the distillation components as the working fluid, (3) overall assessments and (4) experimental data.     

Transient analysis of heat pump assisted distillation systems. 2 Column and system dynamics

A model to simulate the transient behaviour of a heat pump assisted distillation column is presented. The packed bed distillation column is treated as a distributed parameter system with time and space as independent parameters. The column modelling using appropriate basic equations, their application to binary systems, the solution scheme to the model equations and the distillation column simulation algorithm are described. The heat pump simulation algorithm is then coupled with this algorithm and the column transient behaviour with and without heat pump assist is obtained. The results are compared with earlier steady state data in the literature.     

An efficient high-temperature PEMFC/membrane distillation hybrid system for simultaneous production of electricity and freshwater

A new hybrid system model mainly consisting of a high-temperature proton exchange membrane fuel cell and a direct contact membrane distillation is proposed. According to thermodynamics and electrochemistry theories, key performance indicators of the proposed system are formulated, from which feasibilities and effectiveness as well as performance features of the proposed hybrid system are verified. Numerical calculation results indicate that the hybrid system's maximum power density, the according energetic efficiency and exergetic efficiency are, respectively, 5580.3 W m^?2, 49.4% and 23.3%, which are, respectively, 51.8%, 110.9% and 112.0% greater than that of a single fuel cell system. Simultaneously, the corresponding exergy destruction rate density is decreased by 28.1%. Direct contact membrane distillation can be regarded as an efficient waste heat recovery technology. Furthermore, extensive parametric studies show that feed water temperature, flow velocities of both feed water and permeate water, convective heat transfer coefficients of both feed side and permeate side as well as porosity of hydrophobic membrane have positive effects on the hybrid system performance, while hydrophobic membrane thickness and permeate water temperature have negative effects on the hybrid system performance. The results obtained may be beneficial to design and run such a real hybrid system.     

Optimal side heating and cooling in a distillation column

One of the effective methods for saving energy in a distillation column is the use of side heating and cooling. This procedure provides the heat supply at a temperature lower than the reboiler temperature and heat removal at a temperature higher than the condenser temperature, both of which are advantageous from the second-law perspective. The purpose of this paper is to demonstrate the effects of side heating and cooling on separation performance. To treat the ideal case, it was assumed that every plate above the feeder plate releases heat with side cooling and every plate below the feeder plate accepts heat by side heating. It is shown that there is an optimum temperature profile based on both the separation performance and second law. The results are presented graphically by the use of an integrated energy-utilization diagram.     

Heat and mass transfer in vacuum membrane distillation

In the membrane distillation (MD) literature, the heat transfer coefficients of the boundary layers are usually estimated from well known heat transfer empirical correlations developed for non-porous and rigid heat exchangers. A difference between the mechanism of heat transfer in MD systems, which is coupled with transmembrane mass transfer, and the mechanism of heat transfer in “pure” heat exchangers is expected to exist. Vacuum membrane distillation has been experimentally studied in a capillary membrane module and the heat transfer coefficients have been evaluated in both the lumen and the shell side of the membrane module. A critical review of the most frequently used heat transfer empirical correlations in MD systems is presented. Finally, the experimental results obtained in this paper are compared to those of literature, in order to test their applicability in membrane distillation systems.     

Open and closed-cycle mechanical vapour-compression heat-pump assisted sea-water purification systems

The energy requirements for the distillation of impure water can be reduced by using either an open-cycle or a closed-cycle heat pump. The purification of typical sea-water, with a concentration of 3·4 weight %, is considered using an open-cycle mechanical vapour-compression (OMVC) heat pump and also a closed-cycle mechanical vapour-compression (CMVC) heat pump using an external working fluid. Plots are given showing the variation of the thermodynamic properties of sea-water with temperature and concentration. Plots are also given of the energy loads against temperature-lift for both the OMVC and CMVC systems. At temperatures up to 90°C, the work load on the compressor is less for the CMVC than for the OMVC system. The operating characteristics for a number of possible working-fluids for the CMVC system are also considered.     

空气冷却式膜蒸馏构型的传热分析

在膜蒸馏的不同构型中,直接采用环境空气作为冷却媒介的空气冷却式构型很大程度上简化了系统配置。在强化传热的条件下,其跨膜通量与水冷构型接近。对空气冷却式膜蒸馏构型的传热过程进行理论分析,并通过量化分析各参数对膜蒸馏传热性能的影响,构建综合的传热模型。引入关联热阻系数这一概念,用以量化空气冷却的参数对膜蒸馏过程总传热系数的抑制作用。通过模拟计算研究了冷凝板导热系数、空气流速、冷凝板肋化系数、料液温度等参数对膜蒸馏传热性能的影响,并分析和量化多参数对关联热阻系数的综合影响。结果表明冷凝板导热系数、空气流速、冷凝板肋化系数是影响关联热阻系数的重要因素,各参数对膜蒸馏传热性能的综合影响得以量化。以上研究为后续传质模型的研究提供了指导。     

Heat pump assisted distillation. X: Potential for industrial applications

In heat pump assisted distillation, the working fluid can either be one of the components of the mixture to be distilled or it can be an external working fluid. The operating parameters of more than thirty potential heat pump assisted distillation systems have been listed. A detailed assessment can only be made if detailed thermodynamic data are available. A set of criteria has been prescribed for the choice of an appropriate mode for a particular application.     

Heat recovery characteristics of membrane distillation

A simulation model, using a mass transfer coefficient which has as its driving force a humidity difference and a heat transfer coefficient which has as its driving force an enthalpy difference, is developed for investigating the operating characteristics of membrane distillation. Optimum conditions are calculated by the model and the calculated results are proved by actual experiments. Simulated results show that the higher the feed input temperature and the cooling water input temperature, the smaller are the recycling rate, membrane area, and heat energy. For a system which has a 0.216-m² membrane and heat exchange area, optimum conditions of feed input and cooling water input temperature and recycling rate are 90 °C, 50 °C, and 18.5 kg/h, respectively. Furthermore, an actual experiment was performed and its permeate rate and heat input were 1 kg/h and 0.21 kW h. From the work, the latent heat recovery is seen as about 66 percent and the simulation model reliability is verified. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(3): 135–150, 1996     

Theoretical analysis of ammonia-water absorption cycles for refrigeration and space conditioning systems

This paper presents an investigation of an ammonia-water absorption cycle for solar refrigeration, airconditioning and heat pump operations at higher heat supply temperatures. The system consists of a solar driven generator, rectifier, condenser, evaporator, absorber and heat exchangers for preheating and subcooling within the system. A steady state thermodynamic cycle analysis based on mass and heat balances along with the state equations for the thermodynamic properties of the ammonia-water mixture has been carried out. A numerical computer simulation of the system with input component temperatures, refrigerant concentration/mass flow rate and effectiveness of the heat exchangers has been made to evaluate the relative heat transfer rates (i.e. coefficients of performance) and the mass flow rates for the cooling/heating modes. It is found that unlike the low generator temperature behaviour the coefficients of performance for both cooling and heating modes are reduced at higher generator temperatures. However, an increase of condenser temperature for each mode of operation improves the performance of the systems at higher generator temperatures. A choice for keeping the absorber temperature equal to/lower than that of the condenser is also predicted at lower/higher generator temperatures, respectively. In general the results are more pronounced for the refrigeration mode than for the heat pump mode and are least effective for the airconditioning mode.     

A study on membrane distillation by a solar thermal‐driven system

Membrane distillation (MD) is a membrane separation process that has long been investigated in small scale laboratory studies and has the potential to become a viable tool for water desalination. MD is a separation process that combines simultaneous mass and heat transfer through a hydrophobic microporous membrane. A solar collector is used in direct contact membrane distillation (DCMD) to heat seawater as a temperature driving force in heat transfer to establish seawater desalting systems. The effect of the temperature difference makes the brine vaporize in the hot fluid side and condense in the cold fluid side. The optimal operating parameters on the pure water production rate will also be examined in this study. The purposes of this study are to develop the theoretical heat and mass transfer formulations, simulate heat transfer rate of solar collector with internal fins in membrane distillation, and investigate the mass‐transfer efficiency improvement in membrane distillation with the brine flow rate, solar collector efficiency, and temperature difference between both sides of membrane as parameters. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(7): 417–428, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20172     

Transient analysis of heat pump assisted distillation systems. 1 The heat pump

A simple algorithm to simulate the transient behaviour of a vapour compression heat pump is described. Individual models are developed for various components of the heat pump such as compressor, evaporator, condenser and expansion valve. The components are simulated separately and are combined to form the total system. The compressor is a hermetically sealed, reciprocating piston type with adiabatic compression, the evaporator and condenser are coiled copper tubes, and the expansion valve is assumed to be adiabatic. The transport and thermodynamic properties of the refrigerant used are obtained from empirical equations. The total system is viewed in a simplified manner in order to apply the results of this transient analysis to the behaviour of a chemical process operation, namely, distillation.